In order to cope with an increase in demand for communication lines, the wavelength multiplexing number is increased in the wavelength division multiplexing (WDM), and the transmission rate per wavelength is increased in the digital coherent technology. In the wavelength band, the 1.55 μm band (C band) matching the amplification wavelength band of an erbium-doped fiber amplifier (EDFA) is most widely used. A technique using a signal in the 1.59 μm band (L band) by shifting the amplification band of the EDFA to the long wavelength side has also been put to practical use. As a method of further expanding the wavelength band, research and development of optical amplifiers and so forth for use in the 1.46 to 1.53 μm band (S band) is also conducted although such optical amplifiers have not yet been put into practical use.
On the other hand, technology for converting the signal light wavelength by using the four wave mixing (FWM) effect in the optical fiber has been studied and developed for about 20 years. The FWM is a phenomenon in which a new wavelength which does not coincide with any of the incident wavelengths occurs when light of two or more different wavelengths is incident on a highly nonlinear fiber (HNLF). Wavelength conversion techniques using a phase conjugate and the FWM of a nonlinear optical crystal have also been offered. For nonlinear optical media such as nonlinear optical crystals and the HNLF, there is a wavelength at which chromatic dispersion is zero. When the excitation light wavelength coincides with the zero dispersion wavelength, a broadband wavelength conversion is possible.
In a wavelength conversion using the FWM or the phase conjugation, it is possible to collectively convert WDM signals, but there are factors that limit the wavelength band. When the zero dispersion wavelength and the excitation light wavelength do not coincide with each other, the conversion efficiency of the main signal decreases at a wavelength away from that of the excitation light, and a Gain deviation or a tilt occurs. In the following description, both the term “wavelength” and the term “frequency” are used. Since the wavelength and the frequency are in reciprocal relation, the two has practically the same meaning.
FIGS. 1A to 1C illustrate the tilt generated in the main signal light when the zero dispersion frequency and the excitation light frequency do not coincide with each other. FIG. 1A illustrates a simulation of conversion efficiency when the zero dispersion frequency of an HNLF is deviated to the minus side from the excitation light frequency. FIG. 1B illustrates a simulation of the conversion efficiency when the zero dispersion frequency of HNLF is deviated to the plus side from the excitation light frequency. When the frequency deviation is 0 GHz (solid line), a flat conversion characteristic may be obtained over a wide band. As the frequency deviation increases, the signal light whose frequency is away from the excitation light frequency has a low conversion efficiency, and a gain deviation or a tilt occurs. FIG. 1C is a diagram illustrating the amount of degradation of the conversion efficiency of the outermost channel as a function of the frequency deviation. As the frequency deviation increases, signal efficiency deteriorates remarkably on both plus and minus sides.
FIG. 2 is a diagram for explaining the influence of a tilt which occurs in a wavelength conversion. When performing wavelength conversions twice, for example, when converting C band to L band on the transmission side, and then converting the L band to the C band on the reception side, the tilt accumulates, whereby the signal wavelength away from the wavelength of the excitation light is disadvantageous. When the wavelength conversion at a fixed excitation light wavelength is performed by a wavelength converter using a nonlinear optical medium, a tilt occurs in the conversion light as illustrated in FIG. 2. When the excitation light wavelength is controlled to match with the zero dispersion wavelength, the occurrence of tilt is suppressed. However, due to production variations of the HNLF, the zero dispersion wavelength of the wavelength converter on the transmission side may be different from the zero dispersion wavelength of the wavelength converter on the reception side. In this case, the wavelength of the main signal entering the receiver deviates from the ITU grid, and there arises a problem that the main signal may not be received.
The following is a reference document.
[Document 1] Japanese Laid-open Patent Publication No. 2000-75330.